it 'll improve framerate a tiny bit
For what it's worth... (since it's an old thread)
I've the i7 version of this machine with the HD3000 too and did some testing
Cinebench, Xbench, Geekbench, Heaven 4.0
Basically:
-8GB DDR3 1600 CL 9 memory indeed makes the machine faster, the graphics also, but just a tiny bit, it's hardly noticeable (be sure to reset the SMC and NVRAM after doing the upgrade)
16GB 1333 CL9 makes the machine and the graphics also faster by the way
Cinebench OpenGL results
4GB stock: 11,74 fps 384MB graphics men
8GB 1600: 12,08 fps 512MB graphics mem
16GB 1333: 11,90 fps 512MB graphics mem (it does
not get any higher)
So there you have it: a solid 2,9% improvement in framerate

Tested multiple times b.t.w.
I've read @Intel the max memory speed of the HD3000 is about 1300-1350MHz by the way, so probably putting in extra fast memory doesn't work as expected because something goes wrong with the timings.
I haven't noticed a real difference in FCP X for example when editing HD (which is just a bit too much for this type of mac)
There's a 10% bump in overall memory speed though
If there's someone who tested 1866 or even 2133 RAM in this machine please reply to let us hear if it's stable/not to hot/functions properly (or not)